1. Field of the Invention
This invention relates to a method for protecting two static converters with direct-current link from over-currents.
2. Discussion of the Background
Prior art methods for protecting two static converters with direct-current link from overcurrents which known from the Swiss house journal: Brown Boveri Reports 9 (1978), pages 598-601. In this journal, a static excitation device comprising fully controlled antiparallel static converters 9 and 11 carrying circulating current, shown in FIG. 1, is provided for exciting a generator 5 of a hydro-electric power station which feeds a three-phase system 1. The two static converters 9 and 11 are fed from the three-phase system 1 via transformers 15 and 17. In accordance with its polarity and rectifier orientation, the static converter 9 supplies a positive exciter current component I.sub.(+) and the static converter 11 supplies a negative exciter current component I.sub.(-). The exciter current components I.sub.(+) and I.sub.(-) can be detected by means of current transformers 14 and 16 in the feed lines to the transformers 15 and 17. For reasons of better clarity, only one pair of current transformers 14 and 16 each is drawn in FIG. 1; in fact, current transformers are provided in two feed lines to each of the transformers 15 and 17. The current in the third feed line in each case is the result of the sum of the currents in the two other feed lines. Depending on the voltage at the rotor, one static converter always works in rectifier mode and the other in inverter mode.
A firing pulse regulator 4, described in detail in the abovementioned house journal, is connected at the input to the three-phase system 1 via a voltage transformer 2 and a current transformer 3. At the output, it controls the thyristors T1-T6 in the bridge branches of the static converter 9 via a firing angle device 12 and thyristors T1'-T6' in the bridge branches of the static converter 11 via a firing angle device 13.
The field current I.sub.f =I.sub.(+) -I.sub.(-) is fed to the exciter winding of the generator 5 via a de-excitation switch 7. A non-linear de-excitation resistance 8, for example of silicon carbide, in series with a short-circuiter 6 which is provided with anti-parallel thyristors, is connected in parallel with the exciter winding of the generator 5.
A current flow path 34 of the "positive" static converter 9 from 5 via a first contact Of 7, then via T6, 15, T1 and a second contact of 7 to 5 is drawn in dashes. A second current flow path 34, drawn in dots, of the "negative" static converter 11 for the exciter current component I.sub.(-) leads from 17 via T4', T6, 15, T1, a link choke 10 and T3' back to 17. The link choke 10 is used for decoupling the two current flow paths and direct-current circuits 34 and 35 and for preventing short-circuit compensating currents. The compensating current between the two static converters 9 and 11 is regulated to a minimum; it ensures that both static converters are continuously carrying current and that a reversal from negative to positive exciter current can take place virtually without loss of time. Such a reversal is required especially under transient conditions such as, for example, short-circuits of open-circuit power lines. This makes it possible to prevent false disconnections with overvoltages.
This regulating device guarantees stable operation even with zero excitation and prevents oscillation between the two static converters.
For economic reasons, the link choke 10, which is provided with an iron core, cannot be selected to be of any size. In the case of a disturbance caused, for example, by a short-circuit in the three-phase system 1 or in the case of a fault in the static converter regulation, a link current i.sub.Z can rise very rapidly so that the link choke 10 goes into saturation. During this process, the link choke 10 and the "negative" static converter 11 with lower rating can be damaged. Fuses 26, see FIG. 2, in the feed lines to the thyristors T1-T6 and T1'-T6' can become defective. When the link is opened, contacts, not drawn, of a link switch can become welded together. Such overcurrents can also be expected with a correctly operating regulation if overvoltages, out of-balance conditions and voltage distortions occur in the three-phase system 1 as is the case, for example with generators feeding into a disturbed high-voltage direct-current transmission line or if capacitors banks are switched off. In the known method, the generator 5 is disconnected from the three-phase system 1 after a link current has been detected which is too high. The firing pulses are blocked and de-excitation is initiated. Although these measures protect generator and excitation system, the generator is no longer available after the disturbance which, in general, lasts a few tenths of seconds at the most. This outage is particularly undesirable if it affects several machines of a power station.